Low density lipoprotein (hereinafter, referred to as “LDL ”) plays a major role in cholesterol transportation in blood. In particular, most cholesterol deposited on blood vessel walls in the case of atherosclerosis is derived from LDL. An increase in the amount of LDL cholesterol is one of the major risk factors of arteriosclerotic diseases. Thus separate quantification of LDL cholesterol is clinically useful. Moreover, total cholesterol measurement involves measuring cholesterol in all lipoproteins such as chylomicron (CM), very low density lipoprotein (VLDL), LDL, and high density lipoprotein (HDL). Total cholesterol measurement is still a major lipid test.
Conventional methods for quantifying LDL cholesterol include a method comprising two operations (fractionation and cholesterol quantification) and a calculation method using Friedewald's equation based on total cholesterol, HDL cholesterol, and triglyceride levels.
Fractionation includes an ultracentrifugation method, a precipitation method, an immunological method, and the like. These methods require centrifugation or filtration of samples, so that they are currently hardly spread in the field of clinical examination, in light of convenience and economy. Moreover, the calculation method that involves Friedewald's equation is also problematic in terms of accuracy because it does not take individual variability into consideration and the use thereof is limited.
However, recently, a method for quantifying LDL cholesterol that does not require fractionation, has been reported (JP Patent Publication (Kokai) No. 11-318496 A (1999)). This is currently applied for a reagent for clinical examination in the field of examination. This method comprises a first step of selectively erasing cholesterol in lipoproteins other than LDL in a sample (the term “erase” means to decompose ester-type cholesterol and free cholesterol and to make the decomposed products undetectable in a subsequent second step) and a second step of quantifying LDL cholesterol.
However, although the above reagent for measuring LDL cholesterol is a clinically useful, the use of the reagent has not readily become widespread. This is because total cholesterol measurement has been broadly conducted conventionally and LDL cholesterol levels can be obtained by the use of Friedewald's equation. However, as described above, LDL cholesterol levels obtained by the use of Friedewald's equation are problematic. Thus, precise measurement of LDL cholesterol levels has clinical significance. Hence, it has been desired to further improve and diffuse the use of a reagent for measuring LDL cholesterol, which has high clinical significance.
In the meantime, concerning measurement of cholesterol in HDL, a method for continuously measuring cholesterol in HDL and total cholesterol with a single measurement has been reported (M L Sampson et al., Ann Clin Biochem, 37, 479-487, 2000). This method comprises putting a sample in a test tube, measuring HDL cholesterol in the sample using an anti-apoB antibody, disrupting a complex of the anti-apoB antibody and an apoB antibody (HDL cholesterol with the anti-apoB antibody bound thereto) using deoxycholic acid, and then enzymatically measuring the remaining non-HDL cholesterol. The total cholesterol level can be found by totaling values obtained by two instances of measurement. Total cholesterol and HDL cholesterol are conventionally measured broadly in medical checkup and the like. Thus, the ability to measure both cholesterol levels simultaneously is significant.
Patent Document 1
JP Patent Publication (Kokai) No. 11-318496 A (1999)
Non-Patent Document 1
M L Sampson et al., Ann Clin Biochem, 37, 479-487, 2000